Basics of Terraform

Terraform is an IAC (Infrastructure as code) tool that helps to automate provisioning, configuring and managing the application infrastructure, platform and services. 

• It resembles ansible in a major way but ansible is more likely a configuration tool on an existing infrastructure
• We can easily make any changes to existing infrastructure using Terraform.
• We can easily replicate an existing infrastructure using Terraform.

Terraform has two components

• Terraform Core 
• Terraform Input
• Terraform State
• Terraform Providers
• IAAS (Cloud) Providers (AWS)
• PaaS Providers (Kubenetes)
• Service Providers (Fastly)

Terraform core components is used to create plan while the provider components is used to execute that plan.

Terraform code is written in a language called HCL i.e. Hashicorp Configuration Language. The code is saved in a file with extension .tf. It can create infrastructure across variety of providers like AWS, GCP, Azure, Digital Ocean etc.

Terraform Commands

• Refresh: 
• Gets the current state using the provider component
• Plan: 
• Creates an execution plan using the core component
• Apply
• Executes the plan
• Destroy
• Removes the infrastructure

Pre-requisites    

• AWS CLI
• Terraform
• AWS CLI configured for AWS account to be used. See 

Install terraform

• choco install terraform (via Windows Powershell)
• brew install terraform (via Mac terminal)
• Run below command to verify installation
• terraform --version

Terraform plugins

• These are executable binaries written in Go language that communicate with Terraform Core over an RPC interface. e.g. aws provider is a plugin

Terraform  modules

• A module is a container for multiple resources that are used together.
• A terraform configuration has at least one module, known as its root module, which consists of the resources defined in the .tf files in the main working directory.

Terraform providers

• A provider adds a set of resource types and or data sources that Terraform can manage. 
• They are available in terraform registry at url https://registry.terraform.io/browse/providers?product_intent=terraform
• They are constrained in configuration called provider requirements in production environments

# Provider requirements are defined in this block
terraform {
  # Declare the required version using Version Constraint Syntax
  required_version = ">= 1.0"
  # Declare the required providers needed by the module
  required_providers {
    aws = {
      source  = "hashicorp/aws"
      version = ">= 4.50.0, < 5.0.0"
    }
  }
}

Terraform Variables 

• Input
• Input variables let you customise aspects of Terraform modules without altering the module's own source code.
• To declare variables in the root module of the configuration, we can set their values using CLI options and environment variables.
• To declare variables in child modules, the calling module should pass values in the module block.
• An input variable in terraform can be defined as

                    variable "variable_name"{

default = "value",
description="Stores the value for variable_name",
type="string/number/bool/list",
validation{

condition = length(var.image_id) > 4 && substr(var.image_id, 0, 4) == "ami-"

error_message = "The image_id value must be a valid AMI id, starting with \"ami-\"."

}

            }
• 
  

Sample Terraform code

• To define the provider and the region to be used for provisioning infrastructure, you can create a file with name main.tf and add below content

            provider "aws"{

            region = "ap-south-1"

            }

• To create a resource such as instance, database, load balancer etc, you can add content in below syntax

            resource "<PROVIDER>_<RESOURCE_TYPE>" "<RESOURCE_NAME>"{

            [CONFIG ...]

            }

            e.g.

            resource "aws_instance" "testing"{

            ami  = ""

                    instance_type="t2.micro"

            }

• To execute terraform code
• Go to the directory, where the main.tf is saved, via terminal
• Run command
• terraform init
• The above command will initialize backend and the requested provider plugins inside a folder called .terraform
• Run command
• terraform plan -out "myplan.txt"
• The above command will show what terraform will actually do. It is a kind of sanity testing. The plan will be saved to file myplan.txt
• Run command
• terraform apply "myplan.txt"
• The above command will create the resource
• Run command 
• terraform destroy
• The above command will delete all the resources